1. Field of the Invention
This invention relates to a system connectable into standard television receiver systems for converting predetermined channels thereof to audio only channels and detecting selection of the predetermined channels to cause blanking of the picture tube at individual receivers in the system.
2. Brief Description of the Prior Art
In certain types of locations, such as, for example, hotel rooms and the like, it is desirable to provide both radio and television reception for the guests. This type of service has been provided in the past by utilizing individual radio and television instruments. More advanced systems for providing such service with a single unit have been provided as shown, for example, in Nicholson U.S. Pat. No. 4,319,277 and Wheeler U.S. Pat. No. 4,021,849. The Nicholson system, requires that the horizontal sync component be absent. Present day digital scanning receivers require a horizontal sync pulse to sense the presence of a station, thus not allowing the receiver to sense the station presence. Thus Nicholson is not practical for use in conjunction with digital scanning receivers. In Wheeler, blanking of the cathode ray tube is dependent upon an unmodulated picture carrier. The Wheeler circuit requires a major redesign of the television unit for "music only" as obtained by the circuit described hereinbelow. The "music only" switch is an add-on item and requires external power, video sampling from the TV and bias level shift output back to the TV. Wheeler further adds an undesirable feature in that when the picture darkens, the picture tube turns completely black and would therefore return to a bright picture very slowly. These prior art systems have not been versatile in that they cannot be used in conjunction with many present day television systems and are relatively expensive to produce and install.
It is also known that a video black burst generator signal alone will not black out an ordinary television receiver. Preset values of brightness and contrast generally dictate where the level of brightness will be at any given time. Conversely, a high quality studio monitor has d.c. restoration and thus will black out when sent a black burst transmission, however such monitors are very expensive and not generally sold as consumer items. It is therefore apparent that there is a need for a television system which is capable of providing audio to provide, for example, relaxing music, with a totally blackened out picture tube. The system must also be relatively inexpensive and not require monitor facilities and a separate FM radio.